1. New or old, the mask lens needs to be
clean. White toothpaste (I like Colgate
) works best. New masks typically take several "treatments" with toothpaste before you can even hope to keep them fog-free during the dive. Be patient. Spend at least 10 minutes scrubbing, rinsing, and scrubbing. If you have not used your "old" mask in a few weeks, it should be "toothpasted" again.
2. Commercial defog, spit, or for you
really old timers, spit
plus tobacco, all work on a correctly "prepared" mask.
3. The key is leaving just the right amount of "film" on the lens to allow the condensed water to run down the lens in a transparent sheet and not fog...
Me: Spit, rub, quick rinse, then (hope this does not gross out those with weaker stomachs) I
lick the inside of the mask lens, put on the mask, and dive. Just the right amount of "slime" for a fog-free dive
Seriously... it does work, but you do get a lot of weird stares....
Best wishes.